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A Strong Crown at Didipio Mine Leads to Recovery

Hear how the AusIMM Adelaide Branch September Technical Lunch went and the innovations being employed in the Philippines

The Carrington Function Centre was packed out once again for another fascinating AusIMM Technical Lunch. Jason Hodge, the current secretary of the Adelaide Branch and the Principal Mining Engineer at OceanaGold’s Didipio Mine in the Philippines presented on the challenges and methodology behind strengthening and extracting the Crown Pillar at Didipio.

But before we heard from Jason, we had a great presentation from the current Sir Frank Espie/Rio Tinto Leadership Award Winner, Max Donaldson, on the current progress of his Honours Research Project. Max is currently investigating novel collectors for the flotation of high sulphide Cu-Au ore at Carrapateena. The specific collector Max highlighted was the Tecflote S11 collector, which is characterised by a different attachment mechanism and its insolubility in water, hence the air bubble distribution through the pulp is crucial. The use of Tecflote S11 is reported to increase the selectivity of Cu-Au minerals during sulfide ore flotation, which is very useful for Carrapateena due to the high pyrite content of certain sections of the orebody. Preliminary results presented have shown increased selectivity of copper for the same overall recovery (as compared to a typical Xanthate collector) as well as a lower total sulfur recovery, which is interpreted as less pyrite in the concentrate. But when looking at how Tecflote compares to Xanthate with regard to gold, there is a better grade offered initially by Tecflote S11 but at the expense of recovery, the addition of an extra rougher stage does not help, but the addition of a depressant improves the recovery of gold, but does not perform as well as the traditional Xanthate. In order to see the final results of Max’s project, you can attend the final year presentation at Ingenuity in late October.

Once lunch was served, Jason took the podium and talked to us about his experience with strengthening the Crown Pillar at Didipio. Didipio is a high-grade gold-copper mine operated by OceanaGold, the mine started its life as an open pit operation before the recent transition to underground “sub-level open stoping” in 2017. With a current life of mine predicted at 2032, the mine planning team has been considering their options to maximise the recovery of their ore. In particular, how they will access the high-grade ore positioned in the crown pillar of the mine. The crown pillar of a mine is the ore sitting directly beneath the bottom of the open pit, this ore is generally difficult to access due to regulatory restrictions regarding how close you can mine to the bottom of the pit. In the case of Didipio, approximately 20% of their remaining recoverable metal is contained within the crown, meaning it is crucial to engineer a method of extracting this value.

Current regulations in the Philippines allows for mining activity to approach the bottom of the pit within 25 metres if the rock mass is considered sound and impermeable, or 50 metres if these conditions are not met. The problem faced at Didipio is that a section of the pit floor is made up of breccia host rock, which is not considered impermeable and susceptible to chimney failure, causing a 50 metre minimum distance to be imposed on mining operations, leaving a great deal of value in the ground.


Guest speaker Jason Hodge, Branch Chair Chris Sykes and student speaker Max Donaldson

The first stage of engineering at Didipio aimed to strengthen the weak rock mass in the crown pillar, which involved removing the breccia section from the bottom of the pit, but they can’t remove all of it…so what do they do to mitigate the risks of a weak rock mass? OceanaGold decided to strengthen the pillar by replacing the weaker mass with a cemented rock fill (CRF) engineered beam. This helps to eliminate the caving geohazard, eliminate the risk of water inrush and provides a safe working environment to all underground workers.

As of November 2018, the breccia ore has been mined from the open pit and backfilled with CRF, with initial strength testing averaging 1,700 kPa, well above the 1000 kPa theoretical target. An additional 10 metres of paste fill was poured as an impermeable cap on top of the engineering beam, with an aim to provide protection to the beam and mitigate water seepage.

These measures have stabilised the crown at Didipio and satisfied the regulatory requirements, allowing stoping within 25 metres from the top of the engineered CRF beam, unlocking more value from the crown pillar ore.

But this is not the end of the story for the Didipio crown pillar, with the success of their strengthening project, there is now attention focused on the ore located closer to the pit wall, where there is not 25 metres of CRF coverage, this ore still remains inaccessible. There is also concern of intensifying damage in the crown pillar due to repeated exposure to blasting. This culminative damage could lead to the re-introduction of chimney failure in the monzonite host domain.

To address this issue, there is an opportunity to extend the CRF beam coverage across the entire pit floor early in the life of mine. This would involve replacing the monzonite rock mass in the crown pillar with a massive CRF beam, mitigating the risk of chimney failure. Currently at Didipio, a 5 metre strip of the rest of the pit floor has been completed, removing the potentially damaged monzonite rock. With the next step to extend the CRF beam across the entire pit floor and raise the level further, which will allow access to ore closer to the pit walls, previously inaccessible due to the 25 metre restriction.

This pioneering CRF procedure in use at Didipio to strengthen the crown pillar is helping OceanaGold unlock value earlier in the life of mine and eliminating the likelihood of progressive weakening, providing a safer and more reliable underground environment.

All in all, September’s technical lunch was yet another fantastic event that engaged the audience, with both presenters fielding many questions relating to their presentations. Even after official proceedings had concluded, both Max and Jason were caught in deep conversation. Finally, I would like to thank Heathgate Resources for the great opportunity to attend the lunch and would advise all members should pencil in the upcoming November technical lunch.

Brayden Sturm – Heathgate Complementary Ticket Attendee


Branch Chair Chris Sykes & Heathgate complementary ticket winner Brayden Sturm


Branch Partners 2019


Technical Lunch Partner 2019

Media Partner 2019

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